(a) Field of the Invention
The present invention relates to a rear conversion lens system and, more particularly, to a rear conversion lens system for single-lens reflex cameras.
(b) Description of the Prior Art
The lens system disclosed in Japanese published unexamined patent application No. 123515/83 is known as a rear conversion lens system which resembles the rear conversion lens system accoridng to the present invention.
Said known rear conversion lens system comprises a front lens group and a rear lens group wherein the front lens group comprises a first lens arranged as a positive lens, a second lens arranged as a negative lens and a third lens arranged as a positive lens, and the rear lens group comprises a fourth lens arranged as a negative lens and a fifth lens arranged as a positive lens or comprises a fourth lens arranged as a positive lens and a fifth lens arranged as a negative lens. Said known rear conversion system is arranged that the surface on the image side of the first lens has strong curvature. Therefore, spherical aberration of the third order is largely caused at the surface on the image side of the first lens (the second surface) as shown in FIG. 8A. To offset said spherical aberration, said known lens system is arranged that the surface on the object side of the second lens (the third surface) has strong curvature so that the aberration coefficient of spherical aberration of the third order becomes larger as shown in FIG. 8A.
In case of said known lens system, the aberration coefficient of spherical aberration of the third order is large as described in the above and, therefore, the coefficients of spherical aberrations of the fifth and seventh orders are also large as shown in FIGS. 8B and 8C respectively.
As the magnification of said known lens system is 1.4, it is possible to control so that spherical aberrations of higher orders caused at certain surfaces are offset by spherical aberrations of higher orders caused at other surfaces. However, in case of a lens system with a magnification higher than 1.6 like the lens system to be provided by the present invention, it is difficult to arrange that spherical aberrations of higher orders caused at respective surfaces offset each other in the state that spherical aberrations caused by respective surfaces are left large.